148 replies to this topic

[rhetfield]

Can anyone explain the relationship between apparent field of view and true field of view and the importance of either and which matters most?

Apparent field of view is how wide an eyepiece can see.  The ones that came with my scope were 50 degree.  My current ones are 60 degree, so they see more.  True field of view is what they really see.  My 25mm 60 degree EP sees 2.3 degrees true field at 26x.  My old 25mm, 50 degree EP saw 1.9 degrees true field at 26x.  My 5mm 60 degree EP sees 0.5 degrees at 130x.  If either of these were put into your larger scope, the magnifications would be greater and true field of view smaller.

[vtornado]

Looking for anyone’s opinion, input, or preference for viewing planets with a 60 degree AFOV vs a wider 82 degree AFOV?

You may get a bit more drift time with the 82.  82/60 = 40%  However off axis abberations

increase as one moves off axis, so the view in the last 11 degrees of eyepiece may not be as clean as the center 60.

 

Check eye relief, some widefield sacrifice eye relief.

 

I had an ES82, and it was just as sharp on planets and moon than an dual ed.

 

Astigmatim is not that important with small exit pupils.  I have mild astigmatism

and it does not negatively impact the view until 4 or 5mm exit pupil.

For planets I do not wear my glasses.

[Nightskyman]

Thanks, I appreciate everyone’s continuing feedback and find it informative and very helpful. In all due respect to JohnBear’s comment … “anyone’s opinion, input, or preference" really doesn't count!”, it does to me and most other beginners as well. After all, I thought that was the whole point of this beginners forum section. Are we not supposed not supposed to read the books that have been suggested either? we may ask silly questions sometimes, but that’s because we’re starting off with no experience. Varying points of view are not meant to be copied, but rather used to assist in making an informed decision. Back to the task at hand, when saying that wider views may offer some blurred viewing on the outer edges, but is sharp in the center, do you find it a worthwhile trade off for the longer period that the object remains in view?

[Nightskyman]

At this point, due to wearing eyeglasses and having a fair astigmatism, the eye relief will be one of my top concerns with making a choice. Especially since I understand that the stated specifications for eye relief aren’t necessarily correct in reality. 

[Dave Mitsky]

Back to the task at hand, when saying that wider views may offer some blurred viewing on the outer edges, but is sharp in the center, do you find it a worthwhile trade off for the longer period that the object remains in view?

Your telescope has a focal ratio of f/5.9.  I wouldn't worry about eyepiece astigmatism, if you buy an 82-degree eyepiece that is corrected for it.  I would avoid 70-degree 5-lens element Erfle and Erfle variant designs, however.

Mirror coma won't be much of an issue either.

[Dave Mitsky]

At this point, due to wearing eyeglasses and having a fair astigmatism, the eye relief will be one of my top concerns with making a choice. Especially since I understand that the stated specifications for eye relief aren’t necessarily correct in reality.

Part of the problem, other than outright "exaggeration", is that not every manufacturer measures eye relief in the same manner.

[Nightskyman]

Dave, if it wasn’t that my eyes have a moderate astigmatism, I would not even wear glasses when viewing as otherwise I am only farsighted. That is my only concern with the eye relief. I do have a few basic eyepieces, but I really would like and intend to upgrade to something better without breaking the bank. I am comparing the Celestron X-cel LX, AT Paradigm Dual ED ( or Agena Starguider Dual ED), and the AT UWAs. I also currently have an Orion high power 2x 4 element Barlow. In a nutshell, this is what I am trying to decide on. Other than getting a optical finder scope ( likely a RACI), to replace the red dot which is impossible to use with it’s location on my scope and no available space to move it to because of the phone cradle on the top of the tube for the StarSense, I am all set to go! As far as choosing sizes, if I understand correctly, after factoring the focal length of the scope, I choose pieces based on the magnification that I want and the AFOV that I prefer. The only other thing that I don’t quite understand is what the difference is between a fast or slow scope? Not which ratios are which but rather what is the difference with the viewing? Wow, that was a mouthful, but it summarizes the only concerns or questions that I have at this point. Sorry for being so windy and hope the long post isn’t bad etiquette.

[Dave Mitsky]

Dave, if it wasn’t that my eyes have a moderate astigmatism, I would not even wear glasses when viewing as otherwise I am only farsighted. That is my only concern with the eye relief. I do have a few basic eyepieces, but I really would like and intend to upgrade to something better without breaking the bank. I am comparing the Celestron X-cel LX, AT Paradigm Dual ED ( or Agena Starguider Dual ED), and the AT UWAs. I also currently have an Orion high power 2x 4 element Barlow. In a nutshell, this is what I am trying to decide on. Other than getting a optical finder scope ( likely a RACI), to replace the red dot which is impossible to use with it’s location on my scope and no available space to move it to because of the phone cradle on the top of the tube for the StarSense, I am all set to go! As far as choosing sizes, if I understand correctly, after factoring the focal length of the scope, I choose pieces based on the magnification that I want and the AFOV that I prefer. The only other thing that I don’t quite understand is what the difference is between a fast or slow scope? Not which ratios are which but rather what is the difference with the viewing? Wow, that was a mouthful, but it summarizes the only concerns or questions that I have at this point. Sorry for being so windy and hope the long post isn’t bad etiquette.

I was addressing the astigmatism that is present in wide-field eyepiece designs that don't have additional lens elements to correct for it, not any astigmatism present in your vision.  Eyepieces like Erfles work fine in slow telescopes but not so great in fast ones.

https://skyandtelesc...scope-eyepiece/

 

Disregarding issues like coma, central obstruction, and chromatic aberration in refractors, images at the same magnification should appear about the same in fast and slow telescopes of the same aperture and optical quality.

 

While an eyepiece's focal length determines the optical system's magnification, the f/number is of little importance visually. A "fast" telescope implies a short focal length and a large field. Fast, however, is a term borrowed from photography (an f/5 telescope can take a photograph with one-fourth the exposure time of an f/10 instrument).
 

Visually, well-made fast and slow telescopes of the same aperture have no difference in image brightness or resolution. I find that photographers have the most difficulty understanding this concept, because their experience that a faster f/number means brighter images on film and in the viewfinder is so ingrained.

 

https://skyandtelesc...-magnification/

 

Myth #3: Faster telescopes show brighter images.
This is a misconception carried over from photographic use, where the fast f/ratios do mean brighter images and shorter exposures for extended objects. Telescopes with equal apertures and equal magnifications have the same visual image brightness, regardless of the objective's f/number.
 

Myth #4: Long-focal-ratio telescopes give higher-contrast images.
In general, refractors offer the potential for higher contrast because mirror coatings, by their nature, tend to scatter more light. But when comparing well-made, highly corrected refractors, there is no gain in contrast with instruments of long focal ratio.
 

Reflectors too, if well made and having the same size of secondary obstruction, will have the same contrast at the same magnification regardless of the f/ratio.

https://www.televue....rn=Advice&id=86

[Nightskyman]

If collimation is ok, when you see spikes coming from the object and can’t completely remove them from the object observed, can it just be from turbulence or bad seeing?

[vtornado]

Spikes are from the spider vanes.

[Nightskyman]

What do I do to fix it? I can get it close to normal but slightly over or under focusing then I see the spikes or starbursts

[JohnBear]

Just do a google search on "diffraction spikes" for more detailed info. Note that even Hubble and JWST show diffractions spikes on point sources of light. 

 

To reduce or eliminate them you will need a more expensive telescope or do some tricky reengineering on your current telescope, to eliminate the effects that spider vanes produce if you really want be viewing 'point sources of light'. 

Edited by JohnBear, 10 February 2023 - 12:30 PM.

[CowTipton]

What do I do to fix it? I can get it close to normal but slightly over or under focusing then I see the spikes or starbursts

Nothing without changing the secondary spider vanes.

It's part of the design of a reflector telescope.

 

Use the diffraction spikes to your advantage.  When focusing, try to get them as thin as possible.  That's usually your best focus point.

[Nightskyman]

do they primarily show on the brighter stars and less on others?

[gnowellsct]

Apparent field is what you get when you hold the eyepiece up to the sky without putting it in a telescope.  It's the chunk of sky that the eyepiece will let in.  

 

Once the eyepiece is conjoined with a telescope the scope is gathering light and the eyepiece is magnifying it.  Your "true field" is the amount of sky you see in the scope.   It's useful because now you can estimate the size of various objects and assess which eyepiece is best suited to the size of the object.   

 

Greg N

[Nightskyman]

When setting your dob outside to cool down for viewing, is it best to leave dust cap on the end of tube or take it off? I realize the tube must cool down inside, I only ask not knowing if leaving it off while not viewing unnecessarily allows for any dust/ particles to enter. Over the course of time, is there anything that needs to be done to remove any dust that might have accumulated inside the tube? I am not talking about removing or cleaning the mirror.

[Asbytec]

Does leaving it off during observing allow unnecessary dust to enter? I guess the point is, it doesn't hurt much to leave it off because it's going to be off while you are observing anyway. I leave mine off with the fan blowing. But more to your question, it's the mirror that needs to cool. So long as air is blowing on it, then it will cool. It would be nice to exhaust some of that warm air out of the tube. One option is to put the dust cap on and leave the focuser open. You may be shocked how much air blows out of your focuser when the dust cover is on. Try it. Surely enough to keep any dust from floating in. 

[Nightskyman]

When viewing a very bright stage of Venus, does it create diffraction rings as a star would?

[Nightskyman]

What are some of the best 8” dobs that anyone likes?

[drneilmb]

What are some of the best 8” dobs that anyone likes?

I hear lots of good comments on here about the Aperture AD8 and the Orion XT8. There are some other brand names that will also have models very, very similar to those two.

[Dave Mitsky]

What are some of the best 8” dobs that anyone likes?

The 8" Orion Skyline Dob is a bit more upscale than the 8" Orion SkyQuest XT8.

 

https://www.telescop...pe/p/113094.uts

 

There are also the 8" Sky-Watcher 200P Dob, the 8" Sky-Watcher Flextube 200P Collapsible Dob, and the 8" Celestron Starsense Dob.

 

https://www.skywatch...er-classic-200p

 

https://www.skywatch...r-flextube-200p

 

https://www.celestro...onian-telescope

 

The 8" Zhumell Z8 Dob apparently is currently not available.

 

https://zhumell.com/...ector-telescope

[Palmsnbananas]

For me Jupiter is best at 10mm to 5mm, I like 7 or 9mm focal length. 

At higher mags it becomes too blurry

My Focal length is 1200mm so probably same as yours

[SteveG]

At this point, due to wearing eyeglasses and having a fair astigmatism, the eye relief will be one of my top concerns with making a choice. Especially since I understand that the stated specifications for eye relief aren’t necessarily correct in reality. 

What is the CYL on your viewing eye (from your prescription)?
 

From this we can tell you at what magnification your astigmatism will affect the view. Typically, it’s only lower powers. High power views typically have a small enough exit pupil that no astigmatism is seen.

[SteveG]

What are some of the best 8” dobs that anyone likes?

IMO all current Chinese dobs are of equal optical quality. It’s just a matter of mechanical features and accessories. It’s an incredible time to buy quality telescopes & accessories as compared to when I started this hobby.